1. Field of the Invention
The present invention relates to a bearing unit including a bearing formed by disposing lots of rolling elements between an inner ring and an outer ring and an axial force transmitting direction shifting means shifting a transmission path of an axial force, and a main spindle device of a machine tool, which is provided with the bearing unit.
2. Description of the Related Art
In a bearing of a type having lots of rolling elements disposed between an inner ring fitted to a rotary shaft and an outer ring fitted to a housing side, for example an angular ball bearing, an axial force acting on the rotary shaft is transmitted from the rotary shaft to the housing side via the inner ring, the rolling elements (balls) and the outer ring. In that case, when the axial force exceeds a predetermined allowable axial load, there arises a problem that an indentation is generated in an orbital plane, and the like.
With respect to a bearing for a main spindle of a machine tool, a large axial force (unclamping force) acts due to a tool exchange and so forth. When this unclamping force exceeds the above-described allowable axial load, the above-described problem is possibly caused. Therefore, in order to avoid the above-described problem, among conventional main spindle devices of machine tools, there is one provided with an unclamping force support mechanism so as to prevent the unclamping force acting on between the main spindle when exchanging the tool from acting on between the inner ring, the outer ring and the rolling elements (see Patent document 1).
[Patent document 1] Japanese Patent Application Laid-Open No. Hei 4-019413
The above-described mechanism of supporting the unclamping force needs a structure not interrupting rotation of the main spindle at a normal operation but supporting the unclamping force to the main spindle surely at the time of the tool exchange, causing problems of increasing costs due to the complicated structure as well as requiring a relatively large space to be disposed.